At present most of the output from the leather industry is derived from the tanning of animal substrates (e.g. hides) using chromium III containing tanning agents to form an intermediate leather substrate. This intermediate tanned substrate is also known as ‘wet blue’. Wet blue is typically re-tanned, dyed and finished. Chromium III is a particularly effective tanning agent and provides leathers which perform exceptionally well, however there is an increasing level of concern regarding the toxicology and environmental safety of tanning agents containing chromium III compounds.
Attempts have been made to provide tanning agents and processes which are “free-of-chromium” (FOC) and one such FOC option is referred to as “wet white”. Generally, “wet white” is a material obtained by tanning pickled hides/skins with pre-tanning agents such as as aldehydes (e.g. glutaraldehyde), aluminium (III) based tanning products or synthetic tanning agents (syntans).
Unfortunately, in many industrial applications, leathers derived from wet-white processes do not perform as well as chromium III tanned (wet blue) leathers. In particular, the hydrothermal stability, of wet white leather is substantially lower than that for chromium III tanned leather. Generally, for wet white leather substrates, the shrinkage temperatures of the final leathers do not exceed 90° C., and are typically in the region 70-80° C. Thus wet-white based leathers are not commonly used for applications where hydrothermal stability is needed e.g. in steam forming as is often used in the manufacturing of shoes.
The performance requirements of automotive leathers can be especially stringent. Automotive leathers can be required to have greater hydrothermal stability and resistance to photochemical and/or thermal ageing in many demanding environments. Such requirements are likely to become more important to the high-performance automotive leather in the future.
It has also been observed by the present inventors that known tanning processes tend not to provide conditions which facilitate suitably deep penetration of the tanning agents into the substrate, especially when small amounts of fluid (e.g. water) are employed.
In view of the foregoing there is a need for a method of tanning an animal substrate using an efficient, environmentally kind process, utilizing a chromium-free tanning agent which provides a resulting substrate with one or more of the following desirable properties:                1. High hydrothermal stability relative to those normally obtainable with wet white processes;        2. High photochemical/thermal resistance relative to those normally obtainable with wet white processes;        3. High levels of incorporation of the tanning agent;        4. Deep penetration of the tanning agent into the substrate.        
It is also an object of the present invention to provide a method which achieves sufficiently tanned animal substrates more rapidly.
The present invention has as its objectives the provision of a tanning process, resulting tanned animal substrates and final articles comprising the tanned animal substrates which addresses, at least in part, one or more of these needs.